Fig. 2 Grain yield and N harvest index (NHI) for winter wheat (a) and summer maize (b) under different treatments. The NHI values for each of the combined treatments are the averages over three years (Y). Red letters denote significant differences in NHI, blue letters denote significant differences in the grain yield. Different capital letters indicate significant differences among average values of the N at P < 0.05. Different lowercase letters indicate significant differences among the four P treatment at the same N rate. ** and *** indicate the significance at P < 0.01 and P < 0.001 level.